Optical sensors for pressure measurement may be generally divided into two main different families according to the different approach used to measure the pressure parameter: “Interferometric” and “Intensity modulated” optical sensors. In the “Interferometric” optical sensors, the pressure is measured by the phase change between the incoming optical probe beam and the out-coming from an optical pressure sensing element (using Bragg, Fabry-Perot, Michelson, Mach-Zehnder interferometers). In the “Intensity modulated” optical sensors, the pressure is directly measured by the intensity change between the incoming optical probe beam and the out-coming one from an optical pressure sensing element (typically a reflective surface of a pressure sensing diaphragm) In the “Intensity modulated” optical sensors, fiber optic is used to drive incoming optical beam in front of the pressure sensing reflective diaphragm and to collect the light beam reflected by the diaphragm itself.
“Interferometric” sensors have the advantage to measure pressure with an higher resolution than the “Intensity modulated” ones but on the other side, “Interferometric” sensors are more sensitive to mechanical vibrations (often present in industrial environment) and less reliable due to the use of more complicated optical design related to the interferometers. The use of interferometer and the need of a coherent LASER source makes the “Interferometric” sensors even more expensive than the rocky and robust optical “Intensity modulated” sensor where low cost not-coherent LED source may be used.
Optical sensors allow performing pressure measurement in a contactless conditions, making these sensors very interesting for all the applications where fast and periodic pressure changes have to be continuously monitored as in the engine cylinder combustion chamber in the automotive field.
Optical sensors are strongly insensitive to Electro-Magnetic Interference often present in the measuring area in the industrial environment, being the optical probe used to reach the measurement area just based on a fiber optic, which is electrically passive and intrinsically insensitive to EMI problems; for these optical sensors, all the active devices needed to perform optical signal transmission and detection are located far enough away from the pressure measurement area and, typically in a controlled location where EMI problems are not present anymore, eliminating the signal degradation due to EMI or RF Interference.
Pressure measurement performed using an optical sensor in a contactless way and with no need of any active electronic devices in the area where pressure have to be monitored, increases a lot the overall reliability of the sensor itself, making this kind of sensor very appealing for the use in industrial environment where harsh conditions due to extremely high process temperatures are reached as in plastic extrusion, injection and blow molding applications or in automotive applications when pressure measurement in engine cylinder combustion chamber has to be monitored. Said pressure measurement increase a lot the safety in industrial areas with harsh conditions due to the presence of explosive and flammable gas or materials.
Optical sensors using Single Mode Fibers as transmission media, allow performing pressure measurements up to distances of tens of kilometers, making this sensor very appealing for pressure measurement in oil rig, well drilling systems and oil pipeline.
Also the optical sensor allows measuring very high pressure levels because the pressure transducer is based on a deformable pressure sensing diaphragm whose deformation with pressure may be changed modifying its thickness and pressure at high temperature without using mercury (Hg) or other potentially dangerous fluids, and so to be fully compliant with RoHS directive.
Other type of sensors for pressure measurement, the “Piezoelectric” or “Piezo-resistive” pressure sensors, have been developed before the advent of the optical sensors. The physical principle used in “Piezoelectric” electronic sensors is the Piezoelectric effect, shown by some specific crystals (piezoelectric crystals), where a change in the pressure applied to the crystal along a specific direction, produces a voltage change on the crystal itself which is a measure of the pressure applied. In the “Piezo-resistive” electronic pressure sensors, the pressure change is measured by the resistor change induced by pressure typically on a Wheatstone bridge. Even if both kinds of these sensors are widely used in industrial environment where harsh conditions are reached, many of the previous listed advantages are not applicable anymore for this kind of sensors; specifically, this sensor needs a mechanical contact between the transducer chip and the pressure sensing diaphragm. The transducer chip is an active electronic device that needs to be placed very close to the area where pressure has to be monitored (typically few millimeters from the pressure sensing diaphragm); for this reason its reliability is reduced when it works in areas with harsh conditions related to very high temperatures and it needs to be electrically powered. Even if the transducer chip is typically shielded in a metal enclosure avoiding EMI problems, the electronics needed for detection and signal conditioning can still suffer of EMI problems, because it has to be still close enough to the transducer chip box to avoid degradation of small amplitude signal using long and expensive electrical cables. Finally each pressure range to be measured, needs a dedicated design of the transducer chip reducing the possibility to make volume scale economy with the related product cost saving.
U.S. Pat. No. 4,071,753 discloses the general bases of a transducer able to change the optical coupling coefficient according to the acoustic or mechanical energy received; the transducer element is arranged between two different fibers, the first one used as input fiber to provide the incoming optical beam to the transducer element and the second one used as output fiber to receive the optical beam after the transducer element. Changing the optical coupling coefficient according to acoustic or mechanical energy, allows the transducer to convert this energy change into an optical intensity modulated signal. Many other patents disclosed similar inventions based on two or more fiber optic.
U.S. Pat. No. 4,620,093 discloses an optical pressure sensor where a diffraction grating is built on the surface of the deformable with pressure diaphragm; the grating is illuminated by an optical beam from the input fiber optic and the reflected beam from the grating is collected using an output fiber optics ribbon to provide the modulated intensity signal to a photo-position detector able to detect the position change of the diffracted beam. Optical sensor design based on multiple fibers or ribbon is more complicated, more difficult to manufacture with a reliable mass production process and even more complicated to be integrated in smaller size than a single fiber based optical sensor solution.
U.S. Pat. Nos. 5,600,070, 6,131,465, 5,390,546 disclose fiber optics sensor for pressure measurement inside engine combustion chamber using reflective diaphragm and a single fiber approach in order to be integrated in the small site available in the spark plug. Increasing the Signal to Noise Ratio (SNR) and the Sensitivity of the sensor is a very important factor, specifically when small deformations of pressure sensing diaphragm have to be detected measuring the variation of the reflected optical signal. This is more and more important when optical sensor is used to measure high pressure levels (100-1000 Bar) in a reliable way; as matter of fact, when such high pressure levels have to be measured, the pressure sensing diaphragm needs to be thick enough to avoid its breaking with the high pressure action. On the other side, when the thickness of the diaphragm is increased, its deformation range is sensibly reduced and the detection of the intensity variation of the optical reflected signal may be very critical, if a not proper level of SNR and sensitivity are reached. To provide an order of magnitude, 15μm is a typical total diaphragm displacement when a 0-500 Bar pressure range have to be measured in a reliable way using a 1 mm thick steel circular diaphragm with 3.5 mm radius which is mechanically compatible with standard requirement for pressure sensor in extrusion machines; if a 10 Bar resolution is needed, this means the sensor should be able to optically detect 0.3 um displacement.
U.S. Pat. No. 4,678,902 discloses a pressure optical sensor based on a single fiber optic and a reflective diaphragm, where the sensitivity is improved by expanding the light cone coming from a circular shape fiber end surface, further on projected to the reflective diaphragm.
U.S. Pat. No. 5,438,873 discloses a similar pressure optical sensor based on a single fiber optic and a reflective diaphragm, where the improved sensitivity is reached by using a tapered fiber with flat end surface allowing to increase the Numerical Aperture (N. A.) of the fiber on a similar bases of the previous invention.
The prior art optical sensors cannot be used in industrial environment where harsh conditions due to extremely high process temperatures are reached; in fact, this is due to the limited temperature operating range of the available glues and materials needed to optimize the optical design (i.e. coating materials used as anti-reflective layer for refractive index matching to reduce optical back reflections at the fiber optic termination) and to package the optical elements inside the sensor itself (for example the glue materials used to fix optical elements as mirrors, lens, fiber, etc.).
EP 1089062 discloses an optical sensor according to the preamble of claim 1 but wherein the end parts of the waveguides must be parallel one to the other to form a Fabry-Perot interferometer cavity.